This invention relates to field-effect transistors (FETs) and, more particularly, to Group III-V compound vertical FETs.
Excellent material properties such as high electron mobility, high electron peak velocity, low ionization coefficients and good thermal conductivity make InP a good material candidate for FET applications. Among the possible gate technologies for making InP transistors, the insulated-gate technology shows the most promise, and metal-insulator-InP devices with interface state densities as low as 8.times.10.sup.10 cm.sup.-2 eV.sup.-1 have been reported. (see for example, K. P. Pande, Applied Physics Letters, Vol. 46, No. 4, p. 416, (1985)). High performance InP metal-insulator-semiconductor FETs operating both in depletion and enhancement mode (T. Itoh et al., Electron Devices, Vol. ED-30, No. 7, p. 811, (1983)) may find application in high speed digital circuits, in high frequency power amplification (M. Armand et al., Electron Letters, Vol. 19, No. 12, p. 433, (1983)), and, in particular, in integrated optoelectronics for long wavelength optical telecommunication (see, for example, K. Kasahara et al., Electron Letters, Vol. 20, p. 314, (1984 )).
In order to define a finite channel depth, FETs in III-V compounds are conventionally fabricated on semi-insulating substrates. For depletion-mode devices, the active channel is formed either by epitaxial growth or ion implantation, and for enhancement-mode devices by carrier (electron) accumulation on the surface of the semi-insulating substrate. The need for a semi-insulating substrate for such FETs hampers the monolithic integration of photonic and electronic devices, since most of the photonic devices (e.g., laser diodes and PIN photodiodes) are usually fabricated on conducting (e.g., n-type) substrates. Furthermore, the etch pit density of commercially available semi-insulating InP substrates is too high for high quality, high yield optoelectronic device fabrication.